docs: def serialize_footer(signer):
      """Uses the signer object which has been used to sign the message to generate
      the signature, then serializes that signature.
  
      :param signer: Cryptographic signer object
      :type signer: aws_encryption_sdk.internal.crypto.Signer
      :returns: Serialized footer
      :rtype: bytes
      """
      footer = b""
      if signer is not None:
          signature = signer.finalize()
          footer = struct.pack(">H{sig_len}s".format(sig_len=len(signature)), len(signature), signature)
      return footer